Carb not creating vacuum

[Whit]

What would keep my carb from creating a vacuum. My 73 Gt750 will only run in the prime position. I have tested the fuel tap and it will open if a vacuum is connected. Thanks //Pat

[ConnerVT]

If the bike runs, there is vacuum at the intake side of the carbs. the carb itself does not create the vacuum, it is the piston moving downwards in the cylinder that creates the vacuum.

If the fitting to the petcock vacuum line is clear at the carburetor, and there are no leaks in the vacuum line, vacuum is reaching the petcock.

The vacuum from the engine is not a strong, constant vacuum. It is more of a series of pulses of vacuum. depending on how you created vacuum for testing your petcock, there may exist a small vacuum leak (or a binding in the diaphragm mechanism, or an overly strong spring) which is preventing fuel flow when the bike is running, but allows fuel flow during your testing.

[Vintageman]

I think the vacuum comes from only on of three cylinder intakes. It may be possible you have a bad crank seal causing no/low pressure change as piston moves about intake port.

Hate to say that may be wrong of course. You can put you hand over each carb mouth and see if the all seam to feel the same. Mine had a bad seal between two cylinders so only one cylinder sucking real strong.

[ConnerVT]

Yeah, it could be that, too. Though I would figure that if the engine runs (and the complaint is about the petcock, and not the poor running, smokey engine), that suggesting the lower cost/lower effort issues to look at first was most prudent.

[pearljam724]

+1. Bad crank seal. Read and compare your plugs.

[Coyote]

You were just through the motor weren't you Whit?. It's more likely your just on the wrong carb barb. I don't know anything about the 72 - 73 with the individual carbs, But from 74 on, the vacuum fitting is on the center carb I think. 550 is the same way.

[Whit]

Can't be the seals as it only has 15 miles on it since Bill Bune rebuilt the crank. I just put a new OEM petcock on it. I say new but I expect is was made 40 years ago. It could be as was said earlier that the spring is tight. I may put the old petcock on and see if it works as the only problem I had with it was I could not stop the gas leaks for more than a day or two even after rebuilding it. //Pat

[Whit]

Hi Chris,
Yes just rebuilt the engine. The 73 model only has the vacuum on the left carb. This is the same carb that I just patched with JB Weld as it had the plug come out of it. Also the same carb that the slide fell out of and I bent my needle and had to order a new one. This carb has been my problem child. //Pat

[Coyote]

Easy enough to test your new petcock. Put a length of tubing on it and use your mouth. If it's quiet, you can actually hear it doing it's thing..Should not take a lot to over ride the spring inside. If it does, I would take it apart and see why.

[Vintageman]

OK I didn’t know just rebuilt

If I recall correctly, I once took a hose to the petcock vacuum port and suck on the other end of hose like a straw and would see fuel flow from the supply line. I think you can use this technique as a test to see if its petcock or not


Just like Coyote said and his reply beat mine by a few minutes above

Yeah, it could be that, too. Though I would figure that if the engine runs

When I first got my 75 GT750 it ran very well but oil was blowing out of crankcase vent. I even rigged up a PCV from a car to try and stop the oil coming out only when i revved high. I did that stupid thing for the bike seemed to run really well, just the oil shooting out vent when I revved up. I ran the bike for hundreds of mile that way. The other thing you could tell was at idle not all cylinder firing the same... only the one with the better seal idle best. It would idle just a couple of the cylinder you could feel exhaust was week and could not adjust them even

I tried seal rejuvenator trick, didn’t work this time and eventually came out of denial and had it rebuilt.

When I took the carbs off I could feel very low suction. Looking back I am very surprise how well it ran even compared to rebuilt crank and fresh top end

I still have those seals and only thing left was really the metal section. I think it was so bad for bike was left on the side stand for years and had a full tank of gas when he parked it (when he opened tank to show me he was surprise it was empty). I suspect that fuel ran into engine and down hill side seals where fuel pooled literally dissolved those seals

[ConnerVT]

I have tested the fuel tap and it will open if a vacuum is connected. Thanks //Pat

In the original post, Whit said that he had tested the petcock, and that when he applied vacuum, it worked properly.

This is why i said:

If the fitting to the petcock vacuum line is clear at the carburetor, and there are no leaks in the vacuum line, vacuum is reaching the petcock.

With the additional info Whit just gave, my gut says either some JB Weld is where it shouldn't be, blocking the vacuum, or there is a big air leak, leaning the mixture and reducing the vacuum available at the vacuum barb.

[jabcb]

Use a vacuum gauge that's for balancing carbs.
Hook it up to the petcock fitting on the carb & see what you get.

[tz375]

Looking at sets of 72/72 750 carbs, it appears that all three in each set are cast to take a vacuum port, but only the left cab is drilled and has a barb fitting.

Have you checked that it is clear down to the bellmouth and not partially blocked with something?

[Whit]

Petcock is good. I barely suck on the tube and fuel valve opens. I took my plugs out and the spark plug associated with that carb is dry while the others are a little wet. I wonder if the new needle valve I put into this carb has things running too lean. I set it on the 3rd spot like the others. I also feel no vacuum from the carb tube while running (on prime). Taking carb back off too look around. Will make sure nothing is clogged. //Pat

[Coyote]

That's a drag. I'm sure you had visions of everything being perfect and disappearing over the horizon.